My invention concerns interaural time delay of a direct sound superimposed wavefront as it is generated by a loudspeaker array and is perceived by the ears and brain to have a distinct spheroidal propagation and thus, a corresponding radius vector and thus, a psychoaccoustic virtual point-source, hereafter referred to as an image, in three dimensional space.
Space and source perception of human hearing in nature, as well as with reproduced sound, depend concurrently on at least four different parameters of acoustics which are received by the left and right ears and processed in the hearing center in the brain to identify a sound's point-source, not only as to direction, but also in rather exacting distance estimation, i.e. to find the radius vector of a given wavefront.
These four parameters, as long understood, may be listed as loudness (amplitude of a given soundwave); the acoustic ratio (ratio in amplitude of direct to reflected soundwaves); high frequency roll-off (absorption by the atmosphere of energy of shorter wavelengths); and finally, and most significant for image perception, time delay, or the relative difference in times of arrival of a given wavefront (at the same period of phase) at the two respective ears.
In order to explain the physics of creating an image one must note that time delay may be understood to exist in two regions of effect on human hearing. The proportion of the human interaural separation (approximately 15 to 21 cm.), to the audible wavelengths (which vary from approximately 1,720 cm. to 1.72 cm.) may fall into the region referred to as near-field, meaning an interaural phase-shift of time delay which is well within one full cycle of a given wavelength, and which is intelligible by the brain as to degree. On the other hand, this proportion may fall into the region referred to as far field, meaning a phase-shift of time delay which is greater than 360.degree. (one full cycle of a given wavelength), or else very near 0.degree. in the near field which is beyond comprehension to the brain with respect to the oncoming radius vector of a direct wavefront. This far-field proportion is, however, very useful for the spatial reconstruction of reflective walls and other surrounding surfaces in a recorded non-anechoic environment. This use of echo, which may be effective from 10 to 30 ms., is known as the Haas effect and is employed by the recording industry as the primary tool for building a "stereo" as well as "surround" soundstage.
On the other hand a direct oncoming wavefront received by the ears in an anechoic condition, i.e., with no reflective surround echo clues, may be subconsciously measured by the brain as to the phase-shift of the arrival times with respect to the tangent of the wavefront at the two ears. Although the difference may be as little as one tenth of a millisecond, in the near field region (which, with an interaural separation of 15-21 cm., lies between approximately 125 HZ (wavelength=275 cm.) and 1500 HZ (wavelength=23 cm.)), this delay may correspond to a comprehensible amount of phase shift (that is greater than 0.degree. and less than 360.degree.), which may be used to triangulate the angle of the oncoming wavefront to the head, using the following relationship: ##EQU1##
where
.theta. is the arriving angle of the radius vector of the oncoming wavefront; PA1 c is the speed of sound; PA1 t is the time delay; and PA1 x is the distance between the ears.
Furthermore, by slightly "cocking" the head to the first found angle, the brain may refine this estimation in three-dimensional space, subconsciously and nearly simultaneously, triangulating several aspects of the wavefront, and thus, the curvature or radius, ie., with a flatter wavefront signalling a more distant point-source and more rounded wavefront signalling a nearer point-source.